chore: replace xenial with focal in README.md

[toniedzwiedz]

docs: Update README to no longer refer to xenial

Summary

Following the steps in the README.md to the letter leads to downloading the docker image twice. Once for xenial (which is no longer supported by Netlify builds) and once for focal (in Step 2, once ./test-tools/start-image.sh path/to/site/repo is executed).

This commit removes mention of the netlify/build:focal image to align this with step two.

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[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/